 Welcome back mechanically here. Did you know that Poisson's ratio is only one of a few elastic constants that make up the majority of basic problems in strength of materials? This makes us wonder, what are elastic constants? Before we jump in, check out the previous part of this series to learn about what stress and strain are. Now, Poisson's ratio describes a deformation of the material in the directions orthogonal to the applied stress. It is a common observation when a rubber band is stretched since it becomes noticeably thinner. It is denoted by Greek letter U and is mathematically expressed as the ratio of transfer strain over axial strain. Modules of rigidity denoted by G is defined as the ratio of shear stress to shear strain like when cutting a material with dull scissors. Bulk modulus on the other hand describes a material's response to uniform hydrostatic pressure like the pressure at the bottom of the ocean or a deep swimming pool. It is denoted by K. All of these constants are dependent on each other for isotropic materials and are connected via this equation. Factor of safety is a term describing the load carrying capacity of a system beyond the expected or actual loads. Essentially, the factor of safety is how much stronger the system is than it usually needs to be for an intended load. Safety factors are often calculated using detailed analysis because comprehensive testing is impractical on many projects such as bridges and buildings. Hence, we first saw what Poisson's ratio is then moved to what modulus of rigidity and bulk modulus are and then finally saw what the factor of safety is. Thanks a lot for those constructive comments. You helped the channel grow. So, here are the top mechanical years of our last videos. In the next episode of Mechanical Year, find out what volumetric strain is.